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School of Molecular & Biomedical Science |
Blastomyces dermatitidisOn Sabouraud's dextrose agar at 25C, colonies are variable in both morphology and rate of growth. They may grow rapidly, producing a fluffy white mycelium, or slowly as glabrous, tan, non-sporulating colonies. Growth and sporulation are enhanced by nitrogenous substances found in starling dung and yeast extract. Most strains become pleomorphic with age. Microscopically, hyaline, ovoid to pyriform, one-celled, smooth-walled conidia (2-10 um in diameter) of the Chrysosporium type, are borne on short lateral or terminal hyphal branches. On blood agar at 37C, colonies are wrinkled and folded, glabrous and yeast-like. Microscopically, the organism produces the characteristic yeast phase as seen in tissue pathology. B. dermatitidis can be described as a dimorphic fungus because it has both a mould and yeast phase.
Tissue section showing large, broad-base, unipolar budding WARNING: RG-3 Organism. Cultures of B. dermatiditis represent a severe biohazard to laboratory personnel and must be handled with extreme caution in an appropriate pathogen handling cabinet. In the past, conversion from the mould form to the yeast form was necessary to positively identify this dimorphic pathogen from species of Chrysosporium or Sepedonium. However, culture identification by the exoantigen test is now the method of choice.
MIC data is limited. Antifungal susceptibility testing of individual strains is recommended.
Clinical significance:Blastomycosis is a chronic granulomatous and suppurative disease having a primary pulmonary stage that is frequently followed by dissemination to other body sites, chiefly the skin and bone. Although the disease was long thought to be restricted to the North American continent, in recent years autochthonous cases have been diagnosed in Africa, Asia and Europe. All available clinical and epidemiological evidence indicates that humans and lower animals contract blastomycosis from some source in nature. However, the natural habitat of B. dermatitidis has yet to be clearly delineated, despite some reports of its isolation from soil. Mycosis: BlastomycosisFurther reading:Rippon, J.W. 1988. Medical Mycology. 3rd Edition. W.B. Saunders Co., Philadelphia, USA.
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